Fuel cells convert hydrogen and oxygen to water, releasing energy as usable electricity without employing combustion as an intermediate step. Unfortunately, the use of fuel cells has been limited especially where a rapid change in electricity demand is required such as in residential applications. The problem is that the rate of hydrogen supply to the fuel cell must rapidly change in order to accommodate varying electrical loads.
One could use a reservoir of hydrogen from which to supply the fuel cell, and the replenishment of hydrogen to the reservoir would therefore not be subjected to accommodating the rapid changes in hydrogen demand. However, such a solution is impractical, especially for residential use, due to difficulties and risks associated with such storage. Moreover, hydrogen storage equipment adds to the size and cost thereby reducing the attractiveness of a fuel cell for residential use. Alternatively, electricity could be stored in batteries, which serve as a buffer between the fuel cell system and the electrical load. Batteries, especially of the volume required to meet the needs of residential units, also add to the cost and size of the fuel cell system. Moreover, batteries have a limited life and must be replaced. Another approach is to store electricity in a super capacitor. While the size and cost of a super capacitor may be attractive, the disadvantage is the limited storage capacity.
Ideally, hydrogen would be generated on-site on an as needed basis for the fuel cell by the reforming (e.g., steam reforming and autothermal reforming) of fuels such as methanol, ethanol, natural gas, propane, butane, gasoline and diesel. Such fuels have high energy storage densities, have conventional storage protocols and have a nationwide supply infrastructure.
Although technology exists for the generation of hydrogen by reforming fuels, the implemented production processes are not able to quickly change the rate of hydrogen generation so as to be useful in a residential fuel cell application. For instance, hydrogen is widely produced for chemical and industrial purposes by converting suitable fuel material in a reforming process to produce a synthesis gas. Such chemical and industrial production usually takes place in large facilities that operate under steady-state conditions.
On-site hydrogen supply for fuel cells used in smaller mobile and stationary facilities, including residential-scale facilities, poses substantial problems even without the added complexities of operating at varying production rates. For instance, hydrogen generators for fuel cells must be smaller, simpler and less costly than hydrogen plants for the generation of industrial gases. Furthermore, hydrogen generators for use with fuel cells will need to be integrated with the operation of the fuel cell such that energy storage requirements are minimized. Moreover, the hydrogen generators must in combination with the fuel cells, be economically viable both in terms of purchase cost and cost of operation, and they must be sufficiently compact to meet consumer desires.
The challenge associated with providing smaller scale hydrogen generators is readily apparent from the number of unit operations required to convert fuel to hydrogen suitable for use in a fuel cell. The fuel must be brought to temperatures suitable for reforming which are often in excess of 600° C. The fuel is reformed to produce hydrogen and carbon monoxide, and the reformate is subjected to water gas shift at lower temperatures to convert carbon monoxide and water to hydrogen and carbon dioxide. Residual carbon monoxide is removed from the hydrogen-containing gas. Additionally, pre-treatment operations are generally required to treat the fuel to remove sulfur, a catalyst poison.
These unit operations must be conducted in an energy efficient manner. Consequently, the overall process should be highly heat integrated. As can be readily appreciated, changes in hydrogen production would be expected to take some time as each of the unit operations and heat exchange operations respond. The severity of the problem in changing hydrogen generation rates is exacerbated in that the range of operation of residential units needs to be quite wide, often the turndown ratio must be at least 5:1.
The difficulties in providing a hydrogen generator for use with fuel cells is further exacerbated because carbon monoxide is a poison to certain fuel cells such as PEM (polymer electrolyte membrane or proton exchange membrane) fuel cells. The water gas shift reaction is the primary operation used in a hydrogen generator to remove carbon monoxide generated by the reforming of the fuel. Any upset in the operation of the water gas-shift reactor can result in an increase in carbon monoxide that must be removed in downstream treatment of the hydrogen-containing gas. While redundant capacity for carbon monoxide removal (e.g., a selective oxidation) may be used in downstream operations to handle spikes in carbon monoxide production, such an approach will incur a penalty in process efficiency and product purity, as well as compactness and cost of the system. Accordingly, the hydrogen generator must be able to accommodate changes in the hydrogen production rate without adversely effecting the water gas shift operation.
Typically, hydrogen generators are controlled by adjusting the rate of fuel in response to the demand for hydrogen and then measuring process conditions such as burner, reformer and/or water gas shift reactor temperature to control the rate of oxygen-containing gas or water introduction to the hydrogen generator. Similarly, other process conditions can be controlled by measurement and direct feedback to the underlying feed or other process variable. This direct feedback control technique can accommodate the specific design of the hydrogen generator. For instance, direct feedback control of process variables that affect process temperatures such as the rate of oxygen-containing gas feed and water introduction will accommodate heat loss to the environment at variable hydrogen production rates.
Although the use of process condition measurement has met with acceptance, it is not without drawbacks. The two primary disadvantages are slow transient response and instability during transitions, especially rapid transitions. Instability occurs when the direct feedback control results in the operating variable (directly controlled variable) being set too high or too low and the process condition (measured condition), such as temperature, overshoots or undershoots the desired value. Slow response and instability can result in not only loss of efficiency but also can adversely affect the hydrogen product purity and, in some instances, can result in damage to catalyst and equipment.
Copending U.S. patent application Ser. No. 09/815,189, filed Mar. 22, 2001, which is commonly assigned, discloses, inter alia, processes for operating a fuel processor during a transition to a greater hydrogen production rate wherein increased amounts of oxygen-containing gas are provided to the preferential oxidation reactor in anticipation of the increased production rate in order to avoid or attenuate carbon monoxide concentration peaks. This document is hereby incorporated by reference in its entirety.
U.S. Pat. No. 6,267,792 discloses a control apparatus and control method for operating a reformer having a partial oxidation reforming section. The amount of oxygen to be supplied is determined based on an amount of reformate fuel contribution to the partial oxidation reaction, which is determined based on a ratio between a theoretical endothermic value in the endothermic reforming reaction and a theoretical exothermic value in the partial oxidation reaction. The patentees state that the controller determines an amount of time from supply of the raw material to occurrence of the reforming reaction and the partial oxidation reaction, and adjusts the determined amount of supply of oxygen based on that amount of time. The control apparatus may also include a detector for detecting the temperature of the reformer and controlling the supply of oxygen based on the detected temperature so as to maintain the sought temperature with a higher degree of precision. See also U.S. published patent application 2002/0031450.
U.S. Pat. No. 6,322,917 discloses methods for controlling preferential oxidation of carbon monoxide in a reformate stream. In one aspect, the patentees discuss calibrating the fuel cell system at different operating points by determining a target rate for injecting an oxidant into the preferential oxidizer stage for each of the operating points, storing results of calibrating the fuel system in memory, injecting oxidant into the preferential oxidizer stage, while running the fuel system to produce power, at a rate that is determined by the stored results, and while running the fuel system to produce power, periodically, recalibrating the fuel cell to update the stored results.
U.S. Pat. No. 6,565,817 relates to a reformer for a fuel cell. The apparatus disclosed has a load variation detector to detect the variation of a load of a power generating unit of the fuel cell. The detected load variation is supplied to a control device that adjusts a valve for the supply of fuel. The blower controlling air supply for the partial oxidation in the reformer and the valve for controlling the amount of fuel are set on the basis of the calculated air supply and fuel supply.